There is growing pressure for stationary producers of greenhouse gases to dramatically reduce their atmospheric emissions. Of particular concern is the emission of carbon dioxide (CO2) into the atmosphere. One method of reducing atmospheric CO2 emissions is through its capture at a point source and subsequent storage in geological or other reservoirs.
The process for capturing CO2 from power station and other combustion device flue gases is termed post combustion capture (PCC). In post combustion capture, the CO2 in flue gas is first separated from nitrogen and residual oxygen using a suitable solvent in an absorber. The solvent is usually an aqueous basic mixture containing components that undergo a chemical reaction with acid gases such as CO2. It might contain amines (e.g. alkanolamines, ammonia, alkylamines) and/or inorganic salts (e.g. carbonate or phosphate). The CO2 is subsequently removed from the solvent in a process called stripping (or regeneration), thus allowing the solvent to be reused. The stripped CO2 is liquefied by compression and cooling, with appropriate drying steps to prevent hydrate formation. PCC in this form is applicable to a variety of stationary CO2 sources including power stations, steel plants, cement kilns, calciners and smelters.
Acid gases, such as carbon dioxide, are also present in natural gas and other pressurised gas streams and need to be removed to meet gas quality specifications. Pressurised gas streams containing CO2 are also produced in fuel conversion processes such as natural gas reforming and coal gasification combined with a water-gas shift conversion to produce mixtures of hydrogen and carbon dioxide. These gas streams are then suitable for pre-combustion capture of CO2. The conventional approaches for such removal include membrane separation or amine treatment.
When CO2 is absorbed into an aqueous solution a number of reactions can occur. The reactions are shown by the following equations where (1) is hydration of gaseous CO2, (2) is the reaction of CO2 with water to form carbonic acid, (3) is the reaction of CO2 with hydroxide to form bicarbonate and (4) and (5) are the carbonic acid-bicarbonate-carbonate acid-base equilibria.

If an amine, or multiple amines, are present in solution a number of additional reactions may occur. If the amine is a sterically free primary or secondary amine such as monoethanolamine (MEA) or diethanolamine (DEA) the following reactions can occur between CO2 and each amine. Equation (6) is the formation of a carbamate species via a nitrogen-carbon bond formation between the amine and CO2. This is generally the kinetically fastest reaction of those that occur with CO2. Equation (7) is the amine acid-base equilibrium. For polyamines the reactions of equation (6) and (7) may occur for each nitrogen atom. For sterically hindered primary or secondary amines the carbamate species is less stable than in sterically free amines which leads to enhanced formation of the bicarbonate species. For tertiary amines only the acid-base equilibrium of equation (7) occurs.

Monoethanolamine (MEA) is currently employed in industrial CO2 capture but has a number of limitations, including solvent degradation due to oxidation and reaction with nitrogen and sulphur oxides, solvent losses due to high volatility and high energy requirements needed to desorb the CO2 from the CO2 loaded MEA. Some other amines used for industrial CO2 capture have a larger CO2 absorption capacity than MEA, but have poor rates of CO2 capture. Slow CO2 absorption rates are undesirable because to achieve the requisite absorption of CO2, longer contact times between the CO2 containing gas stream and the amine means that longer absorption columns and larger capital costs are usually required.
The use of amines as sorbents in CO2 capture may be limited by the thermal degradation and oxidation of the amines. Much of the research on amine solvents for CO2 capture is based around formulation with commercially available amines. There appears to be little study of novel amines that are designed, via amine structural modification, to match the characteristic requirement of CO2 capture. 4-Aminopiperidine has been reported to perform well in CO2 capture (Singh et al., 2008).
In addition, European patent application no 2036602 (Mitsubishi Heavy Industries, Ltd.) relates to an absorbent liquid for removing CO2 and/or H2S from gas which includes compounds which are described in very general terms in the application (for example, by way of very broad general formulae). However, the data in the application showing the CO2 absorption capacity of the compounds is quite limited.
However, there still exists a need for a more efficient CO2 capture technology or process for post combustion capture.
It is an object of the present invention to overcome or at least alleviate one or more of the problems associated with the prior art.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.